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REVIEW ARTICLE
Microstructural and electrochemical properties of rf-sputteredLiMn2O4 thin film cathodes
K. Jayanth Babu • P. Jeevan Kumar •
O. M. Hussain
Received: 1 November 2011 / Accepted: 29 December 2011 / Published online: 7 January 2012
� The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract Lithium transition metal oxides have received
considerable attention in recent years as high voltage
positive electrode materials in the fabrication of all solid
state microbatteries. Among various lithium-based cathode
materials, LiMn2O4 is one of the most promising cathode
materials as it offers high energy density, high cell voltage,
low cost, and low toxicity over the other electrode mate-
rials. Thin films of LiMn2O4 were prepared by radio fre-
quency magnetron sputtering on gold-coated silicon
substrates under various substrate temperatures ranging
from 373 to 673 K in a partial pressure of 3 9 10-3 mbar
with rf power 100 Watts. In the present investigation, the
influence of substrate temperature on the growth and
microstructural properties was studied. The films deposited
at a substrate temperature less than 473 K was found to be
X-ray amorphous. The initial crystallization has been
observed at a substrate temperature of 523 K. The X-ray
diffraction patterns of the films deposited in the substrate
temperature range 523–673 K exhibited predominant (111)
orientation representing cubic spinel structure with Fd3m
symmetry. The grain size was found to be increased with
the increase of substrate temperature as evidenced from
SEM studies. However, additional impurity phases like
Mn3O4 were observed for the films deposited at higher
substrate temperatures ([673 K) because of re-evaporation
of Li? ions in the films. The electrochemical (EC) studies
were carried for the films deposited at Ts = 673 K in
aqueous media in the potential window of 0.0–1.2 V
exhibited better electrochemical performance suggesting
that the films are well suited as binder free thin film
cathode material for commercially viable Li-ion secondary
batteries.
Keywords LiMn2O4 thin films � Sputtering �Microstructure � Electrochemical properties
Introduction
In the current nano science and technology era, a great deal
of research towards lithium rechargeable batteries is being
carried out by the researchers because of the development
of miniaturized electronic devices such as cellular phones,
notebooks, etc (Komaba et al. 2000). Lithium transition
metal oxides (LTMOs), due to their high electrochemical
potentials, high reversible lithium insertion/deinsertion and
high capacity for lithium storage, are of much interest as a
cathode materials for Li batteries. Among numerous tran-
sition metal oxides, manganese oxide-based compounds are
particularly attractive as cathodes because of their low cost,
abundance, and non toxicity. Among them, spinel LiMn2O4
has emerged as one of the promising candidates because
of its 3D Li? diffusion and high theoretical capacity of
148 mAh g-1 (Thackery et al. 1983; Thackeray 1997). It
crystallizes in normal cubic structure, in which the Li? and
Mn3?/4? ions occupy the 8a tetrahedral and 16d octahedral
sites, respectively, of cubic close-packed oxygen array to
give a cation distribution of [Li]8a[Mn]16dO4. A strong edge
shared octahedral [Mn2]O4 array permits reversible
extraction of the Li? ions from the tetrahedral sites without
collapsing the 3D [Mn2]O4 spinel frame work (Thackery
1995; Manjunatha et al. 2011). The LixMn2O4 provides
relatively high voltage (4 V) in the region of 0 \ x \ 1 and
a plateau at 3 V in the 0 \ x \ 2 region. In the high voltage
K. Jayanth Babu � P. Jeevan Kumar � O. M. Hussain (&)
Thin Film Laboratory, Department of Physics, S.V. University,
Tirupati 517 502, India
e-mail: [email protected]
123
Appl Nanosci (2012) 2:401–407
DOI 10.1007/s13204-011-0054-8
region, it exhibits two step reversible de-intercalation and
intercalation of lithium mechanism. In the first step, lithium
is inserted and de-inserted in a single phase region of spinel
(0.5 \ x \ 1.0), while in the second step a phase trans-
formation from spinel to a k-MnO2 occurs in the range
0.0 \ x \ 0.5 (Amatucci et al. 1999). Nonetheless, the
spinel LiMn2O4 suffers from capacity fading (Mishra and
Ceder 1999; Chiag et al. 2001), which has been a key
problem with LiMn2O4 for commercialization.
It has been observed that the LiMn2O4 in thin film form
shown better cycling stability than its powder counter parts
and can be effectively used as binder free cathode material
in the fabrication of all solid state microbatteries (Fergus
2010). Thin films of LiMn2O4 have been prepared by
several physical and chemical vapour deposition tech-
niques (Hussain et al. 2007; KalaiVani et al. 2007; Shih
and Fung 2006; Shui et al. 2004; Rho et al. 2006a, b). It is
well acquaint that the electrochemical properties of the
electrode materials strongly depends on microstructural
properties which in turn depend upon the type of deposition
technique. Among various physical vapour deposition
techniques, rf-magnetron sputtering technique is observed
to be one of the most favorable and industrially viable
techniques since it enables the formation of homogeneous
films with definite thickness along with good adhesion.
Moreover, the microstructural properties can be altered by
properly controlling the deposition parameters. Numerous
researchers were extensively studied the LiMn2O4 thin
films grown by rf-magnetron sputtering technique (Rho
et al. 2006a, b; Chen et al. 2009; Xie et al. 2008; Komaba
et al. 2000).
Nevertheless, the significance of LiMn2O4 in thin film
form as positive cathode in advance Li-ion batteries
shows impetus scope to carry out the research to inves-
tigate microstructural and electrochemical performance in
the presence of aqueous electrolyte. The study of elec-
trochemical performance in aqueous electrolyte has sev-
eral advantages over non-aqueous electrolytes which
includes low cost and greater safety (Beck and Ruetschi
2000). In addition, the ionic conductivity of aqueous
electrolyte is generally greater than the organic electro-
lyte allowing higher rates and low voltage drops due to
low electrolyte impedance. Dahn and coworkers used
synthesized LiMn2O4 powders as cathode material to
construct aqueous rechargeable coin-type lithium batteries
and the electrochemical properties were first reported (Li
et al. 1994). Furthermore, most of the investigators
studied the electrochemical properties of LiMn2O4 thin
film electrodes in organic electrolyte cells (non-aqueous
electrolyte) (Park et al. 2010), but meager research
reports are available with aqueous electrolyte solutions
(Eftekhari 2001). Hence in the present investigation,
LiMn2O4 films are deposited on metallized silicon
substrates using rf-magnetron sputtering technique and
studied the microstructural, electrical and electrochemical
properties of these LiMn2O4 films as a function of sub-
strate temperature.
Experimental details
LiMn2O4 thin films of about 0.8 lm thick were deposited
from a 3-inch diameter cold pressed and sintered lithium-
rich LiMn2O4 target using rf-magnetron sputtering tech-
nique onto Si/SiO2 (0.3 lm)/Ti (0.03 lm)/Au (0.3 lm)
substrates. The system was evacuated to a base pressure of
less than 5 9 10-7 mbar with a turbo-molecular pumping
system backed by a rotary pump. The distance between the
target and the substrate was maintained constant at 5.0 cm.
During the depositions, the substrate temperature was
varied from 373 to 673 K and the sputtered gas (O2/Ar)
composition was maintained at 1:6 to minimize the loss
of lithium. The rf power applied to the LiMn2O4 target
during sputtering was 100 W and the pressure inside the
chamber while in the process of sputtering was about
3 9 10-3 mbar. A rotary drive mechanism was used to
rotate the substrate holder to obtain good uniformity of the
films.
The structural properties were studied by the X-ray
diffraction technique (Siefert computerized X-ray diffrac-
tometer, model 3003 TT) using CuKa1 radiation (k =
0.15406 nm) source filtered by Ni thin film at a scan speed
of 0.05�/sec in the 2h range 10–60o. The peak positions
were determined precisely using RAYFLEX-Analyze
software. The surface morphological characteristics of the
films have been studied by scanning electron microscope
(SEM) (Carl Zeiss, EVO MA 15).
The electrochemical measurements like cyclic voltam-
metry (CV) and chronopotentiometry (CP) were performed
by designing a prototype aqueous electrochemical cell to
understand the fast transport kinetics of the Li-ions in
LiMn2O4 thin film positive electrode. The design of the cell
(Pt//LiMn2O4) was composed of three electrodes which
were electrochemically suffused in saturated (*7 M)
Li2SO4 aqueous electrolyte media. The rf-sputtered
LiMn2O4 thin film coated on metalized silicon substrate
was employed as working electrode (cathode). A platinum
counter electrode (anode), which acts as a reversible source
and sink of lithium (conducting) ions, and a commercial
calomel reference (Hg/Hg?) electrode, by which the elec-
trochemical analysis was calibrated in the presence of a
saturated Li2SO4 aqueous solution as electrolyte, were
employed. An electrochemical analyzer (model CHI 608C,
CH Instruments Inc., USA) was used for the aqueous cell
measurements and is operated in the cut-off voltage
between 0.0 and 1.2 V.
402 Appl Nanosci (2012) 2:401–407
123
Results and discussion
Structural analysis
The X-ray diffraction patterns of rf-sputtered LiMn2O4 thin
films deposited on metallized silicon substrates at various
substrate temperatures ranging from 373 to 673 K are
shown in the Fig. 1. It is observed that the substrate tem-
perature during deposition significantly affects the micro-
structural properties of LiMn2O4 thin films. A broad and
diffused XRD patterns for the films grown at a substrate
temperature less than 473 K exhibited that the films are
X-ray amorphous. It is noticed that the onset of crystalli-
zation starts for the films deposited at a temperature higher
than 473 K. The degree of crystallization in the films
gradually increased with the increase of substrate temper-
ature (Ts). The grain size is observed to be increased with
the increase in substrate temperature, because as the sub-
strate temperature increases the thermal kinetic energy
increases and hence, the mobility of atoms deposited on the
substrate increases. That means, the possibility of sputtered
atoms settling on a thermodynamically stable position on
the substrate increases by suppressing the nucleation. The
XRD pattern of the film prepared at Ts = 673 K, demon-
strated all the characteristic peaks with predominant (111)
orientation representing cubic spinel structure of Fd3m
symmetry in which Li-ions occupy the 8a tetrahedral sites
and manganese cations occupy 16d octahedral sites of a
cubic close array constituted by the oxygen located in 32e
positions. The full width at half maximum (FWHM) value
of (111) peak was used to assess the film crystallanity.
With the increase in substrate temperature, the crystallanity
of the films was significantly improved.
The lattice parameter for the LiMn2O4 films deposited
at a substrate temperature of 473 K is found to be
8.168 A. The lattice parameter is observed to be increased
with the increase in substrate temperature as shown in
Fig. 2. Higher substrate temperatures generally provide
sufficient kinetic energy for the rearrangement of sput-
tered atoms to grow polycrystalline films with higher grain
sizes. As the particle size increases, the lattice of the
deposited film expands and the volume of the film
increases. Moreover, the higher surface thermal energy
changes the valence state of transition element. This may
be the reason for the increase in lattice parameter. The
observed variation in lattice parameter calculated from the
resulted predominant (111) peak deposited at different
substrate temperatures is an important indication in
determining the nature of spinel phase in the film, i.e.
whether the film has a perfect spinel formation or defec-
tive spinel formation. At Ts = 673 K, the evaluated lattice
parameter was found to be 8.229 A. The reported lattice
parameter of the LiMn2O4 spinel structure is 8.24 A. The
lattice parameter calculated for the films deposited at
lower temperature was much deviated from the ideal value
because of the presence of compressional stresses in the
films. The observed stress components are noticed to be
decreased with the increase of substrate temperature. The
Mn–Mn and Mn–O interatomic distances are also calcu-
lated by considering oxygen parameter (u) as 0.265
(Ohzuku et al. 1990). The Mn–Mn and Mn–O interatomic
distances were also observed to be increased from 2.871
and 1.90 to 2.908 and 1.925 A, respectively, with the
increase of substrate temperature from 473 to 673 K as
Fig. 1 The X-ray diffraction pattern of LiMn2O4 thin films deposited
at different substrate temperatures a Ts = 373 K, b Ts = 573 K, and
c Ts = 673 K
Fig. 2 The lattice constants of LiMn2O4 thin films as a function of
substrate temperature
Appl Nanosci (2012) 2:401–407 403
123
shown in Fig. 3. The increase in Mn–Mn and Mn–O
distance in the cubic lattice structure of the films with the
increase of substrate temperature may provide enough
channels for the lithium ions to have fast intercalation and
de-intercalation. The films deposited at a Ts = 673 K are
observed to be nearly stress free state with reference to the
calculated lattice parameter and Mn–Mn and Mn–O
interatomic distances which are nearly equal to the
observed value (Shinshu et al. 1997). Additional impurity
phases like Mn3O4 were observed for the films deposited
at higher substrate temperatures i.e., Ts [ 673 K which
may be due to re-evaporation of Li? ions in the films.
Surface morphology
The surface morphology of LiMn2O4 thin films was stud-
ied by SEM measurements. Figure 4 displays the SEM
images of LiMn2O4 thin films deposited at different sub-
strate temperatures. It is observed that the surface mor-
phology of the deposited films at a substrate temperature
less than 473 K consists of flat surface with amorphous
nature. The surface morphological features are noticed to
be predominant at Ts [ 523 K. A marked improvement in
the grain size is observed with the increase of substrate
temperature. The LiMn2O4 films prepared at Ts = 673 K
exhibited vertically elongated grains which are seen to be
in good contact with each other. The grain size of the films
was increased with the increase of substrate temperature
and found to be around 250 nm for the films deposited at
Ts = 673 K. At a higher substrate temperature, the
mobility of adatoms enhances on the surface, which leads
to overcome the potential energy of the nucleation sites on
the substrate with net increase in the diffusion distance. In
addition to this, the collision process initiates the nucle-
ation and favors the island formation in order to grow
continuous film with larger grain size. The porosity is also
found to be increased with the increase of substrate tem-
perature which is one of the major properties influencing
the electrochemical property. The electrochemical active
surface area is greater for the films with higher porosity
i.e., the area of contact between the electrode and the
electrolyte is greater hence improved electrochemical
performance can be achieved.
Electrochemical properties
The films grown at lower substrate temperatures
Ts \ 523 K showed no obvious redox process, which is the
characteristic of amorphous nature of LiMn2O4 thin films.
Figure 5 shows the cyclic voltammogram recorded at a scan
rate of 0.5 mVs-1 for LiMn2O4 thin film deposited at a
substrate temperature of 673 K. The peaks located at 0.62
and 0.93 V during cathodic scan corresponding to the Li ion
de-intercalation from LiMn2O4 host matrix to form k-
MnO2, while the peaks located at 0.60 and 0.79 V during
anodic scan correspond to Li ion intercalation into k-MnO2
to form LiMn2O4. This two step de-inserted and inserted
kinetics of Li ion during oxidation and reduction reactions
indicates the characteristic property of spinel LiMn2O4 (Li
and Dahn 1995). In the spinel LiMn2O4, lithium ions
occupy tetrahedral (8a) sites, Mn ions occupy (Mn3?/Mn4?)
octahedral (16d) sites and O2- ions occupy (32e) sites. The
oxygen ions form a cubic close-packed array, tetrahedral
(8a) sites share face with vacant octahedral sites (16c), so
that they form a 3D vacant channels. Lithium ions can
intercalate/de-intercalate through these channels during the
electrochemical reaction (Okubu et al. 2010). The first
oxidation peak (O1) at 0.62 V is attributed to the removal of
lithium ions from half of the tetrahedral sites, whereas the
second oxidation peak (O2) at 0.93 V is due to the removal
of lithium ions from the remaining tetrahedral sites. The
Fig. 6 shows the first discharge curve of LiMn2O4 thin film.
It can be seen that the discharge curve for the film have two
distinct potential plateaus, which is in agreement with the
reduction potentials observed from the cyclic voltammo-
gram. The upper plateau region of the discharge curve
represents a two-phase equilibrium between k-MnO2 and
Li0.5Mn2O4, whereas the second plateau represents phase
equilibrium between Li0.5Mn2O4 and LiMn2O4. For the first
cycle, it is found that the discharge capacity for the films
was 44 lAh cm-2 lm-1, and for the 10th cycle the dis-
charge capacity was 36.7 lAh cm-2 lm-1 as shown in the
inset of Fig. 6. Hee-Soo Moon et al. deposited LiMn2O4
thin films on stainless steel substrates using rf-magnetron
sputtering and reported a discharge capacity of 24 lAh
cm-2 lm-1 (Moon and Park 2002) and Singh et al. (2002)
deposited LiMn2O4 thin films using PLD and reported a
Fig. 3 The variation of Mn–O and Mn–Mn distances with substrate
temperature
404 Appl Nanosci (2012) 2:401–407
123
discharge capacity of about 115 mAh/g for the first cycle.
Platinum can act as an excellent current collector/conductor
during intercalation and de-intercalation reactions. But it is
not an ideal source/sink to be used as an anode in the
electrochemical cell and it may be one of the reason for the
low cyclic retention of the Pt//LiMn2O4 cells (Jeevan
Kumar et al. 2010).
Conclusion
LiMn2O4 thin films with good adhesion have been fabri-
cated on metallized silicon substrate by rf-magnetron
sputtering technique. The influence of substrate tempera-
ture on the microstructural and electrochemical properties
of LiMn2O4 thin films was studied. The degree of
Fig. 4 The SEM images of the
LiMn2O4 thin films deposited at
a Ts = 473 K and
b Ts = 573 K and c Ts = 673 K
Appl Nanosci (2012) 2:401–407 405
123
crystallization of the films gradually increased with the
increase of substrate temperature. The films deposited at a
substrate temperature of 673 K exhibited characteristic
peaks corresponding to (111), (311), (222) and (400)
orientations of the spinel structure with the cubic Fd3m
symmetry. The lattice parameter increased from 8.123 to
8.229 A and the Mn–Mn and Mn–O interatomic distances
were also observed to be increased from 2.871 and 1.90 to
2.908 and 1.925 A, respectively. Typical changes in surface
morphology of the films were observed by SEM with the
increase of substrate temperature. The slow sweep cyclic
voltammetry and chronopotentiometry measurements were
carried out by designing Pt/LiMn2O4 aqueous cell. The
electrochemical features show two well-active redox peaks
that are characteristic of the extraction-insertion of Li? ions
from/into the spinel framework of LiMn2O4. The Pt/
LiMn2O4 electrochemical cell with LiMn2O4 films depos-
ited at a substrate temperature of 673 K exhibited an initial
discharge capacity of about 44 lAh cm-2 lm-1.
Acknowledgments This research work is supported by DST under
Promotion of University in Research Science and Excellence
(PURSE) programme.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution and reproduction in any medium, provided the original
author(s) and source are credited.
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